PROJECT ABSTRACT Our long-term goal is to establish a novel and robust magnetic resonance imaging (MRI) platform that can track a broad range of target molecules to achieve the real-time in vitro and in vivo detection with high specificity and precision. The objective of this R21 application is to demonstrate the potential of 15N2-diazirine as a novel and general tag for MRI. The rationale for 15N2-diazirine as a promising candidate in MRI stems from its unique features: (1) the two 15N-atoms, are not directly bound to protons and are close in chemical shift and strongly coupled, thereby likely delivering a long-lived hyperpolarization state; (2) the diazirine is small in size, known to be readily incorporated into drugs, metabolites and biomolecules, without drastically altering their intrinsic function; and (3) the diazirine is stable and biocompatible in vitro and in vivo conditions. This hypothesis is supported by our preliminary results that 15N2-diazirine compounds can provide long-lived hyperpolarization signal using an experimentally simple and cost-effective hyperpolarization method by SABRE (Signal Amplification By Reversible Exchange). Based on these exciting initial findings, this application will investigate the feasibility and capability of 15N2- diazirines as a universal hyperpolarizable tag to achieve long-lived, enhanced signals for broad applications in MRI. This strategy will overcome the severe limitations in current hyperpolarization MRI by its short time signal (in seconds) and prohibitive instrument cost (in million dollars). The proposed work builds on our existing expertise in synthetic chemistry and chemical biology, as well as our strong collaborations with inorganic and physical chemists on SABRE hyperpolarization NMR. The research in this R21 application will demonstrate the general applicability of 15N2-diazirine as a novel MRI tag that is capable of delivering long-lived signal by a simple and inexpensive hyperpolarization method. The completion of the proposed research also provides a foundation toward our long-term goal of building a transformative MRI platform that can image a broad scope of structurally diverse target molecules in real time with high specificity and sensitivity. New capabilities in MRI obtained from the success of this research are expected to be readily adapted by others and will lead to broad utility and profound impact in biomedical and clinical research ranging from understanding biological processes, tracking dynamic metabolic reactions, to disease diagnosis.